This disclosure is directed to machine or wood screws and is more particularly directed to screws often termed security screws, which are purposely made difficult to remove by unauthorized persons not having the required instrument needed for removal thereof.
Security screws now generally available consist of the following types: one-way screws, drilled spanner, slotted spanner, pinned socket screw, and the Holt screw. The screws are shown, respectively, in FIGS. 1A, B to 5A, B and are indicated as prior art.
The one-way screw, FIGS. 1A and 1B, consist of a standard slotted screw which has the head milled away so that it can only be driven one way. It is a necessarily expensive screw because the head has to be slotted and milled into shape. The head configuration is obtained in three steps: (1) cold heading, (2) slotting, and (3) milling. While the one-way screw is effective, it is expensive and unsightly and very difficult to remove for servicing. It is installed with a regular screwdriver.
The drilled spanner is shown in FIGS. 2A and 2B and the slotted spanner is shown in FIGS. 3A and 3B. They are both, by nature, expensive as the head configurations require either drilling or milling after cold heading. Both screws are installed and removed by using a special spanner wrench. Unauthorized persons can easily remove them by rotating them with a pin punch and hammer.
The hex socket security screw, FIGS. 4A and 4B, is a standard socket head screw which has a pin installed in the center of the socket cavity. It can only be installed and removed by using a hex key wrench that has a hole drilled in the end to slip over the pin. A counterfeit wrench can be made only by annealing a standard wrench, drilling the hole, and rehardening the wrench. The screw can be removed by unauthorized persons by breaking the pin off using a small punch and then using a regular hex key wrench.
These screws are very expensive to manufacture. The head cavity is first broached to its hex shape. A hole is drilled to accommodate.
The Holt screw, FIGS. 5A and 5B, is apparently relatively new to the market. It is installed and removed by using a special socket wrench which has the exact configuration for each screw head size. The screw is driven by the three near vertical slots in the periphery of the binding type screw head.
The screw can easily be removed by unauthorized persons by using a hammer and pin punch to rotate the head. Its cost is moderate compared to the other security screws, but it requires a cold heading operation and a subsequent slot-forming operation to manufacture.
The screw head form described in this disclosure has none of the disadvantages of any of the above screws. It is the most secure because it has no protrusions or cavities in which one might use a pin punch or other tool. Its profile is very low and tapered to thwart the use of pliers. It can only be readily removed by the use of the socket wrench made for its particular size.
Additionally, the preferred embodiments are inexpensive to manufacture since only one cold heading operation is required for their manufacture. No secondary drilling or milling operations are needed.